The present invention relates generally to ink jet printers and, more particularly, to a priming system for priming a printhead during periodic maintenance procedures.
An ink jet printer of the so-called "drop-on-demand" type has at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead, the ink may be contained in a plurality of channels and energy pulses are used to cause the droplets of ink to be expelled, as required, from orifices at the ends of the channels.
In a thermal ink jet printer, the energy pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable by current pulses to heat and vaporize ink in the channels. As a vapor bubble grows in any one of the channels, ink bulges from the channel orifice until the current pulse has ceased and the bubble begins to collapse. At that stage, the ink within the channel retracts and separates from the bulging ink which forms a droplet moving in a direction away from the channel and towards the recording medium. The channel is then refilled by capilary action, which in turn draws ink from a supply container. Operation of a thermal ink jet printer is described in, for example, U.S. Pat. No. 4,849,774.
One particular form of thermal ink jet printer is described in U.S. Pat. No. 4,638,337. That printer is of the carriage type and has a plurality of printheads, each with its own ink supply cartridge, mounted on a reciprocating carriage. The channel orifices in each printhead are aligned perpendicular to the line of movement of the carriage and a swath of information is printed on the stationary recording medium as the carriage is moved in one direction The recording medium is then stepped, perpendicular to the line of carriage movement, by a distance equal to the width of the printed swath and the carriage is then moved in the reverse direction to print another swath of information.
It has been recognized that there is a need to maintain the ink ejecting orifices of an ink jet printer, for example, by periodically cleaning the orifices when the printer is in use, and/or by capping the printhead when the printer is out of use or is idle for extended periods. The capping of the printhead is intended to prevent the ink in the printhead from drying out. There is also a need to prime a printhead before use, to ensure that the printhead channels are completely filled with ink and contain no contaminants or air bubbles. Maintenance and/or priming stations for the printheads of various types of ink jet printers are described in, for example, U.S. Pat. Nos. 5,555,461 and 5,432,538.
In one preferred priming method shown in FIG. 1, a vacuum is created by a diaphragm pump 2 connected to a cap 4 that is brought into sealing engagement with the nozzle face 6 of a printhead 10. An ink accumulator 8 is connected between the printhead and the pump by tubes 11,12 connected to ports 14 and 16, respectively. The accumulator must be formed of materials having characteristics such as sufficient strength to withstand the vacuum force applied by the pump, ease of manufacture, low cost and compatibility with the ink to be collected. One material that is preferred for an ink accumulator is polypropylene, an inexpensive engineering material that is amenable to ease of manufacture. A disadvantage of polypropylene is its very low moisture vapor transfer rate (MVTR). Since the capacity of accumulator 8 is finite, it imposes limitations on the maintenance system to handle waste ink deposited by the priming function. The most obvious limitation is the frequency with which the accumulator must be periodically replaced when filled. Another limitation is that the cap 4 can itself become filled with ink by vacuum pressure reduction or cessation resulting in degradation of quality of output prints formed by the printer.